What should be the proper role of military officers if Il Duce orders a military attack on Greenland or Canada . . .

You are corect in that only Congress can declare war but our constitution and laws are pretty clear that the President can "make" war. Now to answer your question, the military should get involved well before Il Duce orders a military attack and make it clear to The Orange One that they have a duty to "protect and defend the Constitution of the United States against all enemies, foreign and domestic" and The Orange One is a domestic enemy.